This invention relates to a method for electrochemically determining the concentration of heavy metals in water by precipitation of the metals at a solid electrode under the influence of a constant negative d-c voltage, where the water containing the metals is brought into contact with the solid electrode for a specific length of time under constant flow conditions, and by subsequent dissolution of the metals by anodic oxidation, the precipitation process and the dissolution process being repeated continuously. This invention further relates to apparatus for implementing this method.
In the context of environmental protection, the monitoring of industrial waste water, particularly with respect to the content of heavy metal ions, prior to entering a biological purification facility, is of great importance, since poisoning of the live sludge by heavy metal ions, i.e., an inhibition of the biochemical breakdown process, can lead to a lengthy shutdown of the purification plant. Due to their toxicity, heavy metal ions such as those of copper, zinc, cadmium and lead, can furthermore cause damage to the living organisms present in the waters.
The heavy metal content can be determined by various electrochemical methods. At low concentrations, the polarographic methods are particularly well suited for this purpose (see: R. Neeb, "Inverse Polarography and Voltammetry", Verlag Chemie GmbH, Weinheim/Bergstr., 1969, pages 1 to 5). In polarography, the metal ions are reduced and precipitated at a negative working electrode in apparatus including a working electrode, particularly a mercury dropping electrode, a counterelectrode and a reference electrode. The potential of the working electrode is varied at a defined rate and the diffusion limit current is used for the metal determination. In the so-called inverse polarography, an enrichment electrolysis is performed prior to the determination itself, where the metal ions to be determined are precipitated at electrodes of constant surface area at potentials which are more negative than the half-step potentials. The quantity of the precipitated metal depends primarily on the concentration and the duration of the electrolysis as well as, possibly, on the stirring conditions. If, subsequent to the precipitation, the potential of the working electrode is varied at a defined constant rate to anodic values, the metal is anodically oxidized and dissolved again partially at a definite potential. In the current waveform, this manifests itself in a peak which is evaluated.
These polarographic methods, while exhibiting high accuracy and sensitivity, require expensive equipment and have a number of inherent disadvantages. For, an electrolytic liquid of defined composition, i.e., of definite conductivity and a definite p.sub.H value, is required for the determination, and additives must therefore be admixed to the water to be analyzed such as conductive salts and complex formers. In addition, care must also be taken that the temperature is constant. It is further necessary to generate a defined stationary mercury drop before every determination. Since the generation of such drops cannot be automated, polarographic methods are hardly suitable for automatic operation, which is advisable, for instance, for monitoring waste waters and water bodies. Finaly, mercury is also consumed in these methods and the electrodes have only a short life.
In a voltametric method known from U.S. Pat. No. 3,904,487 for determining trace metals, mercury electrodes are likewise used, and specifically in the form of solid electrodes, in which a mercury film is provided on the inside surface of an electrode body of graphite. In this method, the metals, i.e., zinc, cadmium, lead and copper, are precipitated under the influence of a constant negative d-c voltage, i.e., at a potential of -1.4 V (as measured against an Ag/AgCl electrode as the reference electrode). Then, the potential is increased steadily to +0.5 V, whereby the metals are dissolved again at discrete values. During the metal precipitation and dissolution, the metal containing sample solution flows past the two electrodes with a constant flow velocity. The precipitation and dissolving process can be repeated, but the sample solution must be renewed each time. One disadvantage of this method is that unattended operation is not possible, since the electrode surface, i.e., the mercury film, must be recreated prior to every measurement. Furthermore, the detection of and analysis for mercury are not possible, and in addition, mercury containing water accumulates as a waste product with this method, since the mercury film is likewise oxidized after the metal determination proper and is thereby removed from the electrode body.